Enhanced oil recovery (EOR) and improved oil recovery (IOR) operations are sophisticated procedures that use viscous forces and/or interfacial forces to increase the hydrocarbon production, e.g. crude oil, from oil reservoirs. The EOR and/or IOR procedures may actually be initiated at any time after the primary productive life of an oil reservoir when the oil production begins to decline. The efficiency of EOR operations may depend on reservoir temperature, pressure, depth, net pay, permeability, residual oil and water saturations, porosity, fluid properties, such as oil API gravity and viscosity, and the like.
Primary methods of oil recovery use the natural energy of the reservoir to produce oil or gas. Primary methods do not require external fluids or heat as a driving energy. Secondary and tertiary oil recovery methods are used to inject materials into the reservoir that are not normally present in the reservoir, to produce additional oil or gas from the reservoir. Secondary and tertiary methods may be necessary when the primary recovery operation leaves behind a substantial quantity of hydrocarbons in the subterranean formation.
The secondary methods of oil recovery inject external fluids into the reservoir, such as water/and/or gas, to re-pressurize the reservoir and increase the oil displacement. Tertiary oil recovery includes the injection of special fluids, such as chemicals, miscible gases and/or thermal energy. The EOR operations follow the secondary operations and target the interplay of capillary and viscous forces within the reservoir. For example, in EOR operations, the energy for producing the remaining hydrocarbons from the subterranean formation may be supplied by the injection of fluids into the formation under pressure through one or more injection wells penetrating the formation, whereby the injection fluids drive the hydrocarbons to one or more producing wells penetrating the formation. EOR operations are typically performed by injecting the fluid through the injection well into the subterranean reservoir to restore formation pressure, improve oil displacement or fluid flow in the reservoir, and the like.
Suitable injection fluids include, among other things, water, steam, carbon dioxide, and natural gas. In particular, where the subterranean formation contains high permeability zones, the injection fluids may flow through the areas of least resistance, e.g., through the high permeability zones, thereby bypassing less permeable zones. While EOR operations may provide the energy necessary to produce hydrocarbons from the high permeability zones, hydrocarbons contained within less permeable zones may not be driven to production well(s) penetrating the formation.
Examples of secondary and tertiary oil recovery operations include water-based flooding and gas injection methods. Water-based flooding may also be termed ‘chemical flooding’ if chemicals are added to the water-based injection fluid. Water-based flooding may be or include, polymer flooding, ASP (alkali/surfactant/polymer) flooding, SP (surfactant/polymer) flooding, foam injection, low salinity water and microbial EOR; gas injection includes immiscible and miscible gas methods, such as carbon dioxide flooding, and the like. “Polymer flooding” comprises the addition of water-soluble polymers, such as polyacrylamide, to the injection fluid in order to increase the viscosity of the injection fluid to allow a better sweep efficiency by the injection fluid to displace hydrocarbons through the formation. The viscosified injection fluid may be less likely to by-pass the hydrocarbons and push the remaining hydrocarbons out of the formation.
Micellar, alkaline, soap-like substances, and the like may be used to reduce interfacial tension between oil and water in the reservoir and mobilize the oil present within the reservoir; whereas, polymers, such as polyacrylamide or polysaccharide may be employed to improve the mobility ratio and sweep efficiency, which is a measure of the effectiveness of an EOR and/or IOR operation that depends on the volume of the reservoir contacted by the injected fluid. The general procedure of a chemical flood may include a preflush (low-salinity water), a chemical solution (micellar or alkaline), a mobility buffer, and a driving fluid (water), which displaces the chemicals and the resulting oil back to production wells. The preflush composition and the mobility buffer are optional fluids.
When performing a polymer in solution flooding process, a polymer may increase the viscosity of the water to reduce the difference between the oil and the water-based flooding, so that less bypassing or channeling of the floodwater may occur. Said differently, the mobility of the floodwater may be decreased to provide a greater displacement of the flood front. Carbon dioxide (CO2) injection is similar to water flooding, except that carbon dioxide is injected into an oil reservoir instead of water to increase the extraction of oil from the reservoir.
The alkaline/surfactant/polymer (ASP) technique may have a very low concentration of a surfactant to create a low interfacial tension between the trapped oil and the injection fluid/formation water. The alkali/surfactant/polymer present in the injection fluid may then be able to penetrate deeper into the formation and contact the trapped oil globules. The alkali may react with the acidic components of the crude oil to form additional surfactant in-situ to continuously provide ultra low interfacial tension and free the trapped oil. With the ASP technique, polymer may be used to increase the viscosity of the injection fluid, to minimize channeling, and provide mobility control.
It would be desirable to have better compositions and methods for improving the efficiency of an EOR operation.